Ink jet recording methods have become popular because compared to other recording methods, the ink jet recording methods are advantageous in that they proceed with processes with ease and form full color images in a more simple manner, and that images having high resolution can be obtained with devices having simple configurations. Therefore, the ink recording methods have found applications not only in personal use but also in uses in offices, commercial printings, and industrial printings. In these ink jet recording methods, an aqueous ink composition containing an aqueous dye as a colorant is mainly used, but it is poor in water resistance and light resistance. Therefore, a pigment ink containing a water-insoluble pigment has been developed instead of the aqueous dye.
In the inkjet printing used for offices, plain paper is mainly used as a recording medium, and there is a need for attainment of high image density. In general, when a pigment ink is printed on plain paper, the pigment does not remain on the surface of the paper, and penetrates into the paper. Therefore, a concentration of the pigment on the surface of the paper is low, which leads to reduction in image density. When the concentration of the pigment in the ink is high, high image density can be obtained, but a viscosity of the ink becomes high, which leads to reduction in ejection stability.
In order to solve the above problems, various methods for allowing a pigment to remain on the surface of paper have been proposed. For example, there is proposed an inkjet ink containing a liquid vehicle, a colorant, and a polymer, where the polymer has at least one functional group and has a predetermined calcium index value. (see PTL 1). This proposal describes 4-methacrylamide-1-hydroxy butane-1,1-diphosphonic acid as a monomer constituting the polymer. However, this proposal has a problem that storage stability of the ink is low.
Moreover, there is proposed an inkjet recording method where a reception liquid containing a calcium salt is allowed to adhere to paper, to print an image using an ink containing a resin emulsion, a surfactant, a pigment containing a phosphate-containing group (see PTL 2). In this proposal, however, when the ink is printed on plain paper, an effect of improving image density is not sufficiently obtained.
Moreover, similar to the ink jet recording method, there is a problem that an aqueous pigment ink containing a pigment as a colorant is poor in color density on plain paper, particularly white plain paper compared to a dye ink in cases where the aqueous pigment ink is used for writing tools such as felt-tipped markers, ballpoint pens, and plotter pens. In order to solve the above problems, there is proposed an aqueous pigment ink containing urea or derivatives thereof, water, a pigment, an aqueous resin, phosphoric acid ester surfactant, and an O/W type emulsion of copolymer of water-insoluble (meth) acrylic resin and/or stylene-(meth)acrylic acid, which has an average particle diameter in a range of from 50 nm through 200 nm and has a minimum filming forming temperature of 50 degrees Celsius or more (see PTL 3). In this proposal, however, an effect of improving color density is not sufficiently obtained.
In an aqueous pigment ink used for the ink jet recording method and writing tools, it is necessary to stably dissolve a pigment of the aqueous pigment ink in water for a long term, compared to an aqueous dye ink prepared by dissolving a dye in water. Therefore, various pigment dispersing agents have been developed.
For example, as a pigment dispersing agent, a graft polymer containing an aromatic ring at a side chain of the polymer is proposed (see PTL 4).
However, this proposal is insufficient in terms of long-term storage stability. In the ink jet recording method, a hydrophilic organic solvent may be added to the aqueous ink, in order to prevent plain paper from causing curl generated during printing on the plain paper, or in order to improve permeability of the ink to be quickly dried, and to prevent occurrence of beading in cases where the ink is printed on coated paper or art paper. However, when the aforementioned method is applied to the graft polymer described in the PTL 4, pigment dispersiveness is deteriorated and long-term storage stability cannot be secured.